1. Field of the Invention
This invention relates to telephone bridge lifters and, more particularly, to switched impedances for isolating idle ones of a plurality of loops which are connected to a common transmission line.
2. Description of the Prior Art
Two or more subscriber loops are often bridged or connected across a common transmission line to provide party-line service. Such bridging is utilized not only for multiparty service but also for secretarial or answering service and for extension telephones at remote locations. A circuit commonly known as a "bridge lifter" is used to isolate idle loops bridged on the line so as to prevent undesirable loading on the active loops and thereby causing unacceptable transmission loss. When any previously idle loop becomes active by going off-hook, the isolation must be removed and the loop connected to the transmission line without appreciable loss.
Prior art bridge lifters included saturable core inductors such as that described in L. Hochgraf U.S. Pat. No. 2,924,661, granted Feb. 9, 1960. Such inductors are bulky, require substantial mounting space and require associated jumper pairs. An electronic bridge lifter is disclosed in C. E. Jones, Jr. et al U.S. Pat. No. 3,881,068, granted Apr. 29, 1975. This circuit requires the use of high voltage transistors for sensing the off-hook condition and thus is unsuitable for fabrication in integrated circuit form. Moreover, such bridge lifters require at least two resistors bridged across the line to sense a differential current flow and thereby control the switch. These resistors must be moderate in value and contribute substantially to the transmission loss when the lines is in use.